136 BELL SYSTEM TECHNICAL JOURNAL 



mense value, and no theory of the atom can be spared from the demand 

 that it confront them and account for them. 



U. Interpretation of X-Ray Spectr.\ 



By the term "X-ray" the reader may understand any radiation 

 of which the frequency v is so high, that the energy hv of a single 

 quantum is several times as great as the energy required to remove 

 the most-easily-detached electron from an atom; greater, for instance, 

 than 100 equivalent volts, so that the wavelength of the radiation is 

 less than some 125 Angstrom units. The emission or the absorption 

 of such radiation by an atom involves too great an energy-change 

 to be attributed merely to a displacement of the valence-electron or 

 even to combined displacements of the valence-electron and one or 

 two others. This definition leaves a sort of "twilight zone" of radia- 

 tions having frequencies somewhat but not much greater than 1/h 

 times the ionizing-potential of an atom. Little is known about such 

 radiations, and in this place they will not be considered. 



The absorption of an X-ray quantum by an atom results in the 

 extrusion of an electron from the atom. The emission of an X-ray 

 quantum results from the passage of an electron within the residue of 

 the atom from some original situation to the situation vacated by 

 the extruded electron, or else into a situation vacated by an electron 

 which itself has moved elsewhere within the atom. These state- 

 ments contain the theory of the vast amount of data piled up by 

 observations upon the emission and absorption of X-rays by matter. 



To express the same statements rather differently: X-ray absorp- 

 tion-spectra and X-ray emission-spectra reveal, when analyzed for 

 Stationary States in the manner used in analyzing optical spectra, 

 that each atom with several or many electrons has a considerable 

 number of Stationary States, distinguished from those we have here- 

 tofore discovered in that each of them involves the absence of one electron 

 from the atom. Each of them is therefore strictly an "ionized-atom 

 state," and yet there are several of them with extremely different 

 energy-values. This signifies that the extraction of an electron from 

 an atom rich in electrons may leave the residue in any one of several 

 distinct conditions. These distinct conditions are the distinct Sta- 

 tionary States, transitions between which are responsible for X-ray 

 spectra. Owing to this striking difference between the Stationary 

 States hitherto described and these latter, I shall refer to these as the 

 "X-ray Stationary States"— not that this name is a particularly 

 good one. 



